(a) Technical Field
The present invention relates to a membrane electrode assembly with enhanced hydrophobicity and a method for manufacturing the same. More particularly, the present invention relates to a membrane electrode assembly with enhanced hydrophobicity, which maximizes the surface area of a catalyst layer by forming a nano pattern having a high aspect ratio in a surface catalyst support of the catalyst layer to provide superhydrophobicity to the surface of the catalyst layer, and increases hydrophobicity by coating a hydrophobic thin film on the surface thereof.
(b) Background Art
In an electrochemical reaction for generating electricity in a Polymer Electrolyte Membrane Fuel Cell (PEMFC), hydrogen supplied to an anode that is an oxidation electrode is separated into electrons and protons that are hydrogen ions. Protons move to a cathode that is a reduction electrode through a polymer electrolyte membrane, and electrons move to the cathode through an external circuit. In the cathode, oxygen molecules, protons, and electrons react with one another to produce electricity, heat, and water as by-products.
A Membrane Electrode Assembly (MEA) used in PEMFC generally includes a polymer electrolyte membrane and a catalyst layer of the anode and the cathode. The polymer electrolyte membrane may include Per-Fluorinated Sulfonic Acid (PFSA) and hydrocarbon ionomer with various structures.
The catalyst layer generally includes a catalyst including a single metal using platinum (Pt) as a base material or a binary or ternary alloy, a catalyst support for supporting the catalyst, and a binder used for mixing the catalyst and the catalyst support. Particularly, as a catalyst support, carbon powder, such as carbon black, that can be stably used under a PEMFC operation environment due to its sufficient electrical conductivity, specific surface area, and durability is typically used.
Examples of such carbon powder include but are not limited to Vulcan® XC72R and Black Pearls® 2000 from Cabot® Corp, Ketjenblack® EC300J and Ketjenblack® EC600JD from Ketjen Black International, Shawinigan Black® from Chevron®, and Denka Black® from Denka®. Also, in recent years, studies on reaction surface increasing materials using Carbon Nano Tube (CNT) and Carbon Nano Fiber (CNF) as catalyst supports for increasing the performance and the durability of a fuel cell, and other materials such as Nano Structured Thin Film (NSTF) are being extensively conducted.
When water produced during the electrochemical reaction in a fuel cell appropriately exists, water plays a desirable role of maintaining the humidity of the polymer electrolyte membrane. However, when there is a surplus of water and the water is not appropriately removed, “flooding” may occur at a high current density. This flooding may hinder reactant gases from being efficiently supplied to the fuel cell, causing a greater voltage loss.
Particularly, when water is produced by an Oxygen Reduction Reaction (ORR) in the cathode of MEA, and the MEA catalyst layer has hydrophilicity or low hydrophobicity, water produced in the ORR may not be appropriately discharged, causing a mass transport loss which restricts smooth supply of atmospheric oxygen to the electrolyte membrane and thus reduces the performance of the fuel cell. Also, when water produced in the cathode continuously increases compared to that of the anode, back diffusion of water may occur from the cathode to the anode, causing flooding in the anode as well and thus hinders hydrogen from being supplied to the electrolyte membrane through the anode catalyst layer.
Accordingly, it is desirable for both cathode and anode catalyst layers to have high hydrophobicity so that water produced from the electrochemical reaction of the fuel cell can be smoothly discharged.
While typical materials such as carbon black, CNT, CNF, and NSTF can allow the MEA catalyst support to have a certain level of hydrophobicity, the MEA catalyst layer manufactured using the MEA catalyst support generally have a contact angle of 150 degrees or less with respect to water, and mostly have hydrophobicity of 120 degrees to 140 degrees. Accordingly, there is a limitation in that the water discharge performance of the MEA catalyst layer is not as superior as it should be or needs to be.
Also, it was reported that hexafluoroethane (C2F6) Radio Frequency (RF) plasma etching has been performed on carbon black. In particular, Vulcan XC-27 powder used as a catalyst support to introduce Trifluoromethyl (CF3) functionality into the surface, and thus the contact angle of carbon black itself was increased from 70 degrees to 156 degrees.
However, the contact angle of actual catalyst layer when surface-treated carbon black is used for manufacturing the catalyst layer of an actual MEA, and stability of maintaining the surface structure in the MEA catalyst layer were not yet reported. Also, since the plasma surface treatment was performed on the original carbon material of the catalyst support, and the carbon material is additionally allowed to be a catalyst layer, the hydrophobic surface structure of the catalyst support may not be sufficiently realized in the actual MEA catalyst layer.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.